1. Field of the Invention
This invention relates to improvements in a catalyst deterioration diagnosis system for an internal combustion engine, arranged to diagnose a deteriorating condition of a catalyst by using two air-fuel ratio sensors which are disposed upstream and downstream of a catalytic converter including the catalyst.
2. Description of the Prior Art
A variety of catalyst deterioration diagnosis systems for an internal combustion engine have been proposed and put into practical use. A typical one of these deterioration diagnosis systems is disclosed, for example, in Japanese Patent Provisional Publication No. 63-205441, in which two air-fuel ratio (or oxygen) sensors are disposed respectively upstream and downstream of a catalytic converter (including a catalyst) of the engine so that diagnosing deterioration of the catalyst is accomplished in accordance with comparison of output signals from the respective sensors while an air-fuel ratio feedback control is accomplished mainly in accordance with the output signal from the upstream-side oxygen sensor.
More specifically, during execution of the air-fuel ratio feedback control, fuel supply to the engine is controlled mainly in accordance with the output signal from the upstream-side oxygen sensor, for example, under a false proportional plus integral control so that the output signal from the upstream-side oxygen sensor periodically repeats inversion between a low oxygen concentration side and a high oxygen concentration side relative to a slice level corresponding to a stoichiometric air-fuel ratio as shown in FIG. 7A. This exhibits that the concentration of residual oxygen on the upstream side of the catalytic converter largely changes. In contrast, on the downstream side of the catalytic converter, change of the residual oxygen concentration is mild under an oxygen storing capability of the catalyst and therefore the output signal from the downstream-side oxygen sensor is as shown in FIG. 7B in which the change of the output signal is small in amplitude and long in cycle.
When deterioration of the catalyst in the catalytic converter proceeds, the oxygen storing capability of the catalyst is lowered so that the oxygen concentrations on the upstream and downstream sides of the catalytic converter becomes similar to each other. As a result, the output signal from the downstream-side oxygen sensor repeats its change in a cycle similar to that of the output signal from the upstream-side oxygen sensor, increasing its amplitude, as shown in FIG. 7C. Accordingly, with the catalyst deterioration diagnosis system of Japanese Patent Provisional Publication No. 63-205441, an inversion cycle T1 of the output signal from the upstream-side oxygen sensor between the low and high concentration sides and an inversion cycle T2 of the output signal from the downstream-side oxygen sensor between the rich and lean sides are measured to determine a ratio (T1/T2) between the inversion cycles T1 and T2. Then, when this ratio exceeds a predetermined level, it is judged that the catalyst has been deteriorated.
Another catalyst deterioration diagnosing system is disclosed in Japanese Patent Provisional Publication No. 4-1449, in which a catalyst is finally judged to have deteriorated when a condition of a ratio (f2/f1) exceeding a predetermined value is detected a predetermined plurality of times. The ratio (f2/f1) is between a frequency (f1) of inversion between high and low oxygen concentration sides of an output signal from an upstream-side oxygen sensor and a frequency (f2) of inversion between lean and rich sides of an output signal from a downstream-side oxygen sensor.
However, drawbacks have been encountered in the above conventional catalyst deterioration diagnosis systems, which will be discussed hereinafter with reference to FIGS. 8A and 8B.
FIG. 8 shows at its upper part an example of the change of the output signal from the upstream-side oxygen sensor between the low oxygen concentration side and the high oxygen concentration side, and at its lower part a change of a feedback correction coefficient .alpha. corresponding to the change of the output signal. The feedback correction coefficient .alpha. is obtained, for example, by the false proportional plus integral control. More specifically, when a curve of the output of the oxygen sensor crosses the line of the slice level corresponding to the stoichiometric air-fuel ratio so as to be inverted from the low oxygen concentration side to the high oxygen concentration side, a predetermined proportional amount PL is added to the feedback correction coefficient .alpha. and additionally an integral amount with an inclination corresponding to a predetermined integration constant IL is gradually added to the feedback correction coefficient .alpha.. This feedback correction coefficient .alpha. is raised to a basic fuel injection amount as well known. Thus, actual air-fuel ratio is gradually made rich (in fuel). Then, when the curve of the output signal from the oxygen sensor has been inverted from the high oxygen concentration side to the low oxygen concentration side, a predetermined proportional amount PR is subtracted from the feedback correction coefficient .alpha., and additionally an integral amount with an inclination corresponding to a predetermined integration constant IR is gradually subtracted from the feedback correction coefficient .alpha.. Upon repetition of the above operation, the air-fuel ratio is maintained at values near the stoichiometric air-fuel ratio while making a small variation in its level.
Here, assuming that the engine is under a steady state operation, the slice level corresponding to the stoichiometric air-fuel ratio is to reside generally at a vertical center of the amplitude of the air-fuel ratio feedback correction coefficient .alpha. as shown at the lower part of FIG. 8. It is to be noted that times tLR, tRL indicated in FIG. 8 is regarded as a control delay in a feedback control system, each of the times tLR, tRL being a time for which the output signal from the oxygen sensor is actually inverted into its low oxygen concentration side or its high oxygen concentration side upon the periodically changing feedback control correction coefficient .alpha. crossing the slice level corresponding to the stoichiometric air-fuel ratio.
In case that the upstream-side air-fuel ratio (oxygen) sensor has not deteriorated and is normal, an increase and decrease cycle Ta of the feedback correction coefficient .alpha. is relatively short while decreasing the amplitude W as shown in FIG. 9A. As a result, the downstream-side oxygen sensor is hardly affected with exhaust gas passing through the catalytic converter, so that the output signal from the downstream-side oxygen sensor hardly has inversion between the rich side and lean side as shown in FIG. 9D if the catalytic converter is normal.
In contrast, in case that the upstream-side air-fuel ratio (oxygen) sensor has deteriorated to increase a response delay in air-fuel ratio feedback control, the increase and decrease cycle Ta of the feedback correction coefficient .alpha. increases while increasing the amplitude W. In this case, change of the air-fuel ratio is increased over the oxygen storing capability of the catalytic converter, and therefore inversion between the low and high oxygen concentration sides will appear in the output signal from the downstream-side oxygen sensor as shown in FIG. 10D even though the catalytic converter is normal. As a result, there is the possibility of the catalyst in the catalytic converter being erroneously diagnosed to be deteriorated, even though the catalyst has a normal ability or performance.